The present invention relates to monitoring of x-ray exposures in medical diagnostic equipment. It finds application in conjunction with radiographic apparatus, such as fluoroscopic imaging systems, and will be described with particular reference thereto. However, it should be appreciated that the invention will also find application in conjunction with other equipment where precise adjustable positioning of a radiation or visible light detection means is desirable.
Fluoroscopic imaging systems include a continuous source of radiographic energy, such as an x-ray tube, which propagates radiation through an object to be imaged, such as a human patient, onto a screen of fluorescent material, the object to be imaged being disposed in a gap between the x-ray source and the fluorescent screen.
X-ray radiation passing through the imaged object is attenuated according to the density of the material through which it has passed. Radiation passing through dense material, such as bone, will be attenuated more than radiation of similar energy passing through less dense material, such as tissue. For uniformly intense radiation entering an object the radiation energy exiting the object is a reflection of the attenuation occurring within the object. Attenuated radiation impinging on the fluorescent screen is absorbed by the fluorescent material thereon and converted into a relatively low brightness visible light in proportion to the radiation energy impinging at each point thereon. This conversion results in a two dimensional light image of the object represented by a plurality of different intensities of visible light. This light image can be visualized by the human eye or captured onto photographic film, which is generally more sensitive to light than to x-ray. The brightness of the fluorescent screen is sufficient to expose film placed in direct contact with the screen, but the light output is generally too low for direct diagnostic visualization, photographing with a camera, or viewing with a television camera. In many applications, a device is needed that will convert the x-rays into light and intensify, or increase the brightness of, the light. An image intensifier tube is such a device.
The intensifier tube can be described as an evacuated glass bottle. The large area of the bottle forming the bottom of the bottle is the input screen, and the small area that forms the `cap` on the bottle is the output screen. The input screen is comprised of two layers. The first layer which the x-ray beam encounters contains a fluorescent material. The fluorescent material absorbs the incident x-rays and converts a portion thereof into a low level visible light. The light is absorbed by an adjacent photo cathode layer, the second layer of the input screen. The absorption of light by the photo cathode layer results in the emission of low energy electrons into the evacuated bottle. The intensifier tube is connected to an electrical energy source that applies a relatively high bias voltage between the photo cathode and the output screen. The bias voltage accelerates the low energy electrons in the tube towards the output screen. A plurality of electrodes in the intensifier tube steer the electrons towards the output screen. The accelerated electrons, which are now at a relatively high kinetic energy potential, converge on and strike the output screen phosphor which converts the electrons energy into relatively bright flashes of light representative of the radiation image at the input of the intensifier tube.
The input end of a video camera is held in fixed relation to the output of the intensifier tube in order that the output image of the intensifier tube can be viewed by the camera input. The video camera is part of a closed circuit television system which provides a visual image on a television screen, representative of the radiation image detected at the input of the image intensifier.
In fluoroscopy, the patient is exposed to a relatively low intensity source of continuous or rapidly pulsed x-ray radiation so that the radiologist can dynamically view the operation of the internal body structure being imaged on the television screen. In practice a balance is made between minimizing patient exposure to x-ray radiation and the need to provide sufficient radiation to produce a quality diagnostic image. Some factors that will influence the amount of radiation to be delivered to a patient in a particular imaging sequence are; the path length the radiation will traverse within the patient and the attenuation of the radiation within patient structures being imaged. Preliminary selection of x-ray dose rate can be made on the basis of empirical data however, because path length and attenuation are patient dependent variables, the actual effect of x-ray dose selection is not known until the output image is viewed. Further, in some fluoroscopic imaging systems, the x-ray source, image intensifier, video camera and related components are contained within a movable gantry structure which allows the fluoroscopic system to be dynamically positioned about the patient. Also, the patient couch can be moved, relative to the x-ray source and image intensifier, during imaging operations to optimize the image view. If the gantry and/or patient couch is adjusted during imaging the image intensity may change due to fluctuations in the radiation path length or different radiation attenuation characteristics in different portions of the patient. Because the viewed image is capable of dynamically changing due to the reasons set forth above, it is desirable to provide a means to dynamically adjust the x-ray dose rate precisely to maintain the same image quality regardless of changing conditions.
One way to assure consistent image quality is to measure the x-ray dose rate after the x-rays have passed through the object to be imaged. A way to accomplish this is to introduce a light sampling means between the intensifier tube and the video camera. The light sampling means samples a portion of light and directs the sampled light to a sensing and control means which modifies the x-ray dose in response thereto.
One such sampling means is comprised of a pair of mirrors or prisms disposed on a mirror assembly, a light opaque cylindrical housing and a photomultiplier tube (PMT). For the purpose of description the central axis of the housing is vertically oriented. The housing bottom is open and the housing top is closed. A light input hole is disposed along one side of the housing approximately mid-way between the top and bottom. The centers of two evenly distributed threaded screw holes are disposed between the light input hole and the bottom of the housing. The PMT is a vacuum tube device having a light detection array, an input for accepting electrical biasing for the detection array and an electrical output from the detection array. The PMT is snugly received through the bottom of the housing and is oriented such that light propagating through the light input hole will fall on the light detection array. The mirror assembly includes first and second support arms which are adjustably engaged to the housing. The first support arm is secured to the housing, below the light input hole, by projecting two screws through two horizontally oriented oval slots in a first portion of the first arm and securing the screws into the threaded holes in the housing. The horizontally oriented oval slots provide limited rotational movement about a horizontal axis perpendicular to the central vertical axis of the housing. A second, vertically oriented, portion extends perpendicular from the first portion and outward from the housing. A third portion, having two threaded holes for mounting the second arm, extends further outward from the housing and upwards from the second portion. The second arm is secured to the first arm by projecting two screws through an upper circular hole and a lower horizontally oriented oval slot in a first portion of the second arm and into the threaded screw holes in the third portion of the first arm. The lower oval slot provides for rotational movement about a horizontal axis perpendicular to the axis described in conjunction with the two horizontally oriented oval holes of the first arm. A second portion of the second arm extends perpendicular to the third portion of the first arm transverse to the face of the light input hole.
The two mirrors are attached to the second arm such that light propagating upward from the vertically oriented image intensifier tube is reflected generally horizontally by the first mirror to the second mirror which in turn reflects the light at a right angle directly into light input hole.
To align the mirrors to detect light from a point on the output of the intensifier screen, an x-ray mask is disposed between the x-ray source and the intensifier tube input screen. The x-ray mask is a sheet of x-ray transmissive and x-ray opaque pattern portions which causes a known x-ray pattern to be disposed on the intensifier input screen when the x-ray source is engaged. The x-ray pattern striking the intensifier input screen results in a representative light pattern on the intensifier output screen. The above described sampling means is positioned adjacent the video camera such that the first mirror extends between the intensifier output screen and the video camera to sample light from the output of the intensifier tube. The screws projecting through the oval slots of the first arm are loosened to allow rotational adjustment of the first arm thereby resulting in adjustment of where the mirror views the intensifier output screen along a first line which is generally parallel to the second portion of the second arm. While monitoring the electrical output of the intensifier tube, the first arm is rotated until the electrical output of the PMT is optimized. While holding the first arm in its adjusted position the screws are tightened securing the first arm to the housing. Next, the screws projecting through the hole and oval slot in the second arm are loosened to allow rotational adjustment of the second arm thereby resulting in the mirror assembly being adjusted along a second line across the intensifier output screen. The second line is generally perpendicular to the first line. While monitoring the electrical output of the PMT, the second arm is adjusted until the PMT output voltage is maximized. While holding the second arm in its adjusted position the screws are tightened securing the second arm to the first arm. The adjustment of the first and second arms continues iteratively until the PMT voltage output is maximized; this being an indication that the mirrors are centered on the light image created by the x-ray mask.
One problem with using the above described assembly to aim the mirrors is that once the screws in the oval slots are loosened for adjustment the entire assembly is subject to shifting thereby not providing a mechanism for subsequent arm adjustments to progressively build upon prior arm adjustments. Therefore, the adjuster is faced with the possibility of having to begin the adjustment process anew every time the securing screws are loosened. Also, the adjuster may inadvertently vary the alignment before or during the tightening of the alignment screws. Because of the above described problems the alignment of the mirrors tends to be time consuming and tedious.
Another problem with the above assembly is that two pair of securing screws, one pair per arm, at a right angles to each other, require tightening before the assembly is secured. As the sampling means typically resides in a confined space, the tightening of two pairs of screws at right angles is physically difficult.
The present invention contemplates an x-ray sensor with adjustable viewing means which provides an improved adjustment means which overcomes the above-referenced problem and others.